The present invention relates to semiconductor circuits, and more particularly concerns circuits for translating a voltage from one reference-voltage level to another.
It is frequently necessary to translate an electrical input signal having a swing about a first unvarying reference or bias voltage to an output voltage having the same swing about a second unvarying reference or bias voltage. That is, the input signal voltage must be translated from one DC level to another, either without changing the magnitude of the signal swing, or with a precisely known ratio between input and output swings. It is also frequently necessary to sum a number of such translated signals together to form a composite output signal.
Many applications require that the translation and other operations be accomplished very precisely and at very high speed. One application which stresses the state of the art in this area involve shifting an AGC (automatic gain control) voltage referenced to ground to a signal referenced to a 6-volt supply voltage; another is the translation of a DAC (digital-to-analog converter) signal referenced above ground to a signal centered about a six-volt supply voltage.
Typical conventional circuits would translate signals with a combination of operational amplifiers and passive components. Such circuits suffer from reduced bandwidth, offset-voltage errors, and large numbers of components. U.S. Pat. No. 3,778,640 (Platt, et al.) teach a simpler circuit for logic voltage-level translation, but one which still suffers from offset errors, and which requires that the input signal voltage be within a restricted range. The gain through this circuit is highly temperature dependent, and has a wide tolerance.